Inverters have been employed in many types of electrical equipments, as an efficient variable-speed control unit. Inverters are switched at a frequency of several kHz to tens of kHz, to cause a surge voltage at every pulse thereof. Inverter surge is a phenomenon in which reflection occurs at a breakpoint of impedance, for example, at a starting end, a termination end, or the like of a connected wire in the propagation system, followed by applying a voltage twice as high as the inverter output voltage at the maximum. In particular, an output pulse occurred due to a high-speed switching device, such as an IGBT (Insulated Gate Bipolar Transistor), is high in steep voltage rise. Accordingly, even if a connection cable is short, the surge voltage is high, and voltage decay due to the connection cable is also low. As a result, a voltage almost twice as high as the inverter output voltage occurs.
As coils for electrical equipments, such as inverter-related equipments, for example, high-speed switching devices, inverter motors, and transformers, insulated wires made of enameled wires are mainly used as magnet wires in the coils. Further, as described above, since a voltage almost twice as high as the inverter output voltage is applied in inverter-related equipments, it is required in insulated wires to have minimized partial discharge deterioration, which is attributable to inverter surge.
In general, partial discharge deterioration is a phenomenon in which an electrical-insulation material undergoes, in a complicated manner, for example, molecular chain breakage deterioration caused by collision with charged particles that have been generated by partial discharge of the insulating material, sputtering deterioration, thermal fusion or thermal decomposition deterioration caused by local temperature rise, and chemical deterioration caused by ozone generated due to discharge. For this reason, reduction in thickness, for example, is observed in the actual electrical-insulation materials, which have been deteriorated as a result of partial discharge.
It has been believed that inverter surge deterioration of an insulated wire also proceeds by the same mechanism as in the case of general partial discharge deterioration. Namely, inverter surge deterioration of an enameled wire is a phenomenon in which partial discharge occurs in the insulated wire due to the surge voltage with a high peak value, which is occurred at the inverter, and the coating of the insulated wire causes partial discharge deterioration as a result of the partial discharge; in other words, the inverter surge deterioration of an enameled wire is high-frequency partial discharge deterioration.
In order to prevent the deterioration of insulated wires caused by such partial discharge, investigations have been conducted on an insulated wire having a high voltage at which partial discharge occurs. In order to obtain this insulated wire, a method of increasing the thickness of the insulating layer of the insulated wire can be considered.
Japanese Patent No. 4177295 discloses an insulated wire in which an adhesive layer is provided between a baked enamel layer and an extrusion-coated resin layer, and the adhesive strength between the baked enamel layer and the extrusion-coated resin layer is strengthened by using the adhesive layer as a medium. When this technique is used, since the solvent resistance of the adhesive layer is lower as compared to other enamel resins, the mechanical characteristics after solvent impregnation are reduced to a large extent.
Further, attempts have been made hitherto to impart added values in terms of properties (properties other than the partial discharge-occurring voltage) to the enameled wire by providing a resin coating at the outer surface of the enameled wire. For example, JP-A-59-040409 (“JP-A” means unexamined published Japanese patent application), JP-A-63-195913 and the like are mentioned as techniques of the related art in terms of the constitution of providing an extrusion-coated resin layer on an enamel layer. However, these techniques were not so satisfactory in terms of the constitution of the thickness of the enamel layer or the extruded coating, from the standpoint of balancing between the partial discharge-occurring voltage and the adhesiveness between the conductor and the enamel layer.